Fine particulates which adhere to a silicon semiconductor surface can reduce the yield or efficiency of the wafer as can be well imagined. These particles will adhere to one another, creating larger size particles termed agglomerates. The origins of the particles are literally too numerous to list: dust, pollen, flakes of human skin, oxides, etc, as well as debris from slicing and lapping operations.
The primary holding forces are van der Waals forces and electrostatic forces. Chemical bonding may also prevail. Numerous methods have heretofore been proposed for reducing or purging the particles: filtering the air in the production facility, personal fastidiousness, spinning the wafer to centrifuge the particles, immersing the wafer in a liquid to reduce adhesion, and so on. Immersion, however, can introduce another force, namely capillary attraction upon removal of the wafer from the immersion bath.
The foregoing background is set forth in more detail in an article entitled "An Analysis of Particle Adhesion on Semiconductor Surfaces," R. Allen Bowling in SOLID-STATE SCIENCE AND TECHNOLOGY, September 1985, presenting the ultimate conclusion that emphasis should be placed on prevention of particle deposition in the first place rather than relying on subsequent removal efforts.
The article by R. Allen Bowling takes into account an earlier investigation of detergent cleaning, both aqueous and nonaqueous, as a means of removing the offending particles, but this technique did not alter the author's conclusion. Indeed, the author stressed criticality of the size of detergent molecules, which must be small enough to wedge between the offending particles and the silicon surface, meaning that effective removal by detergents would involve relations between the size of the offending particle and the size of the detergent molecule.
Detergents are organic in nature; many are of a polar nature and themselves tend to bond to the wafer chemically as noted in a recent article, "Cleaning Techniques for Wafer Surfaces" (Semi-International, 1987). This same article stresses use of ultrasonics and megasonics as aids in chemical cleaning, deemed especially helpful in loosening polar bonds such as those which can arise from the use of peroxides; for example, ammonium hydroxide-peroxide solutions are employed to break the strong electrical particle bonds.
The 1987 article ccncludes by updating chemical cleaning, also known as wet chemistry. Considerable detail is presented in terms of the complex mechanics employed for wet chemistry (immersion bath equipment, centrifugal spray equipment, and so on). Few details of chemistry are discussed, only generalities for the most part, such as "acids," "oxygen plasmas," "choline chemistry" and "RCS chemistry." Choline chemistry, because of its foul odor presents a handling problem. Therefore it is reluctantly accepted provided a closed system is adopted. The so-called "RCA chemistry" involves two aqueous systems applied in sequence, namely, an NH.sub.4 OH/H.sub.2 O.sub.2 treatment followed by an HCl/H.sub.2 O.sub.2 treatment. The solutions are volatile, giving off noxious fumes which, if they mix, result in settlement of NH.sub.4 Cl particles. Other problems are discussed. Processing the wafer by methods described above depends a great deal upon whether the wafer is one freshly sliced from the rod of crystals on which it grew or whether it is a wafer which has undergone subsequent IC fabrication such as resist coating, photolithography, insertion of conductor pins and so on. Thus, one can compare the disclosure in U.S. Pat. No. 4,159,619 which addresses prefabrication surfactant cleaning of freshly sliced, polished wafers and the disclosure in U.S. Pat. No. 4,276,186 where the concern is with an effort to purge an IC module of solder flux residue and to remove from the chip the so-called top seal material. Many chemicals when used by themselves tend to objectionably discolor and etch the wafer surface; hence great care is required. Discoloration of the wafer is perceived by the electronics industry as a possible source of electrical problems. Among these are potassium hydroxide and choline hydroxide.
The present invention is concerned with the first of these general procedures, that is, cleaning a silicon wafer freshly sliced from the long rod where it was grown. After slicing the wafer, it is ordinarily subjected to rough lapping, and then progressively finer polishing to render the opposed surfaces as flat and parallel as possible. The residues or soil to be removed are "particulate," that is, adherent particles of silicon and polishing materials left over from the lapping and polishing procedures.